Connected Components Parallel Search

The connected_components_ps() function computes the connected
components of a graph by performing a breadth-first search from
several sources in parallel while recording and eventually resolving
the collisions.

The algorithm computes how many connected components are in the
graph, and assigns each component an integer label. The algorithm
then records to which component each vertex in the graph belongs by
recording the component number in the component property map. The
ComponentMap type must be a Distributed Property Map. The
value type must be the vertices_size_type of the graph. The key
type must be the graph's vertex descriptor type.

Every N th nodes starts a parallel search from the first vertex in
their local vertex list during the first superstep (the other nodes
remain idle during the first superstep to reduce the number of
conflicts in numbering the components). At each superstep, all new
component mappings from remote nodes are handled. If there is no work
from remote updates, a new vertex is removed from the local list and
added to the work queue.

Components are allocated from the component_value_allocator
object, which ensures that a given component number is unique in the
system, currently by using the rank and number of processes to stride
allocations.

When two components are discovered to actually be the same component,
a collision is recorded. The lower component number is prefered in
the resolution, so component numbering resolution is consistent.
After the search has exhausted all vertices in the graph, the mapping
is shared with all processes and they independently resolve the
comonent mapping. This phase can likely be significantly sped up if a
clever algorithm for the reduction can be found.